the same time to all the cylinders; or cylinder-individual, in which the engine control unit can adjust
the injection for each cylinder individually.
Internal mixture formation
In an engine with an internal mixture formation system, air and fuel are mixed only inside the
combustion chamber. Therefore, only air is sucked into the engine during the intake stroke. The injection
scheme is always intermittent (either sequential or cylinder-individual). There are two different types of
internal mixture formation systems: indirect injection, and direct injection.
Indirect injection
Air-cell chamber injection – the fuel injector (on the right) injects the fuel through the main
combustion chamber into the air-cell chamber on the left. This is a special type of indirect injection
and was very common in early American diesel engines.
Indirect injection as an internal mixture formation system (typical of Akroyd and Diesel engines); for
the external mixture formation system that is sometimes called indirect injection (typical of Otto and
Wankel engines),
In an indirect injected engine, there are two combustion chambers: a main combustion chamber, and
a pre-chamber, that is connected to the main one. The fuel is injected only into the pre-chamber
(where it begins to combust), and not directly into the main combustion chamber. Therefore, this
principle is called indirect injection. There exist several slightly different indirect injection systems
that have similar characteristics. All Akroyd (hot-bulb) engines, and some Diesel (compression
ignition) engines use indirect injection.
Direct injection
Direct injection means that an engine only has a single combustion chamber, and that the fuel is
injected directly into this chamber. This can be done either with a blast of air (air-blast injection), or
hydraulically. The latter method is far more common in automotive engines.
Typically, hydraulic direct injection systems spray the fuel into the air inside the cylinder or
combustion chamber, but some systems spray the fuel against the combustion chamber walls (M-
System). Hydraulic direct injection can be achieved with a conventional, helix-controlled injection
pump, unit injectors, or a sophisticated common-rail injection system.
The latter is the most common system in modern automotive engines. Direct injection is well-suited
for a huge variety of fuels, including petrol (see petrol direct injection), and diesel fuel.
In a common rail system, the fuel from the fuel tank is supplied to the common header (called the
accumulator). This fuel is then sent through tubing to the injectors, which inject it into the
combustion chamber. The header has a high pressure relief valve to maintain the pressure in the
header and return the excess fuel to the fuel tank.